The invention relates to a process for the preparation of polysuccinimide (PSI) and polyaspartic acid (PAA).
The preparation and use of polyaspartic acid and its derivatives has for a long time been the subject matter of numerous publications and patents. The preparation can thus be carried out by thermal polycondensation of aspartic acid (J. Org. Chem. 26, (1961) 1084).
US-A 4 839 461 (=EP-A 0 256 366) describes the preparation of polyaspartic acid from maleic anhydride, water and ammonia. Maleic anhydride is converted into the monoammonium salt in an aqueous medium with the addition of concentrated ammonia solution. The monoammonium salt is polymerized to polysuccinimide in the melt especially at temperatures of 125.degree. to 140.degree. C. and the polysuccinimide is converted into PAA or a salt thereof by hydrolysis. This procedure may pass through viscous phases which are difficult to control industrially. WO 93/23452 refers to the production of polyaspartic acid from maleic anhydride, water and ammonia too.
It is known from US-A 4 590 260 (=JP-A 1984(59)-60160) to subject amino acids to a polycondensation together with derivatives of malic, maleic and/or fumaric acid at 100.degree. to 225.degree. C. According to US-A 4 696 981, microwaves are employed in such reactions.
DE-A 2 253 190 (=US-A 3 846 380) describes a process for the preparation of polyaminoacid derivatives, specifically polyaspartic acid derivatives. According to this process, in addition to aspartic acid, maleic acid derivatives (the monoammonium salt and monoamide) are also used, by thermal polymerization, for the preparation of the intermediate stage of polysuccinimide, which in turn can be reacted with amines in suitable solvents to give the desired derivatives.
According to EP-A 256 366 (US-A-4 839 461), polyaspartic acid can be employed for removal of encrustations caused by hardness-forming agents in water (scale inhibition and scale deposition removal). According to US-A-5 116 513 and EP-A-454 126, polyaspartic acid and salts thereof are active constituents of detergents and fertilizers.
The invention relates to an improved process for the preparation of polysuccinimide and polyaspartic acid by reaction of fumaric acid, maleic acid or a derivative thereof with urea, isourea, carbamic acid, ammonium carbamide, ammonium bicarbonate, diammonium carbonate and mixtures of the abovementioned substances in a reactor at temperatures of preferably 100.degree. C. to 300.degree. C. over reaction times of preferably 0.5 minutes to 300 minutes, the resulting polysuccinimide being converted, if appropriate, into polyaspartic acid or salts thereof by hydrolysis.
A preferred variant comprises mixing maleic anhydride or maleic acid with one of the nitrogen-containing carbonic acid derivatives described above and a metal carbonate or metal bicarbonate and subjecting this mixture to discontinuous or continuous thermal polymerization at preferably 100.degree. C. to 300.degree. C. over reaction times of preferably 0.5-300 minutes in a suitable reactor, if appropriate in the presence of a solvent.
Polyaspartic acid in the present invention is understood as meaning both the free polyaspartic acid and its salts.
In a preferred embodiment, the polyaspartic acid prepared according to the invention essentially comprises recurring units of the following structure: ##STR1##
The chemical structure is preferably analyzed by .sup.13 C-NMR and, after total hydrolysis, by HPLC, GC and GC/MS.
In addition to the polyimide, the abovementioned recurring units a) and b) can also be present in the product obtained directly during the polymerization.
In addition, by a suitable reaction procedure and choice of starting materials, the product can comprise further recurring units, for example Malic acid units of the formula ##STR2## and maleic acid and fumaric acid units of the formula ##STR3##
If, in addition to the maleic acid derivative and the abovementioned nitrogen-containing carbonic acid derivatives, metal carbonates or metal bicarbonates are employed as a third component, partly neutralized structures can also be present, in addition to the abovementioned structures. The amount of metal carbonate or metal bicarbonate here is chosen such that 10% to 90% of the carboxyl groups possible in the polymer are neutralized. All metal carbonates or bicarbonates which are stable under normal conditions can be used. However, the bicarbonates or carbonates of the alkali metals and alkaline earth metals are preferably to be employed. Particularly preferred cations are Li, Na, K, Mg, Ca and Ba. The partly neutralized polyaspartic acid derivatives show a significantly improved solubility in water compared with pure polysuccinimide. In addition to recurring succinimide units, they contain the abovementioned units a) and b), which can be present in salt form.
The maleic acid derivatives to be employed according to the invention, such as maleic anhydride or maleic acid, are reacted with the nitrogen-containing carbonic acid derivatives according to the invention, such as urea, isourea, carbamic acid, ammonium carbamide, ammonium bicarbonate, diammonium carbonate and mixtures thereof, such that the molar ratio of nitrogen to maleic acid derivatives is between 0.5 and 2, preferably between 0.8 and 1.2 and particularly preferably between 0.95 and 1.05.
The starting material mixtures required for the polymerization can be prepared in a separate process. Thus, the mixing operation can be carried out in bulk in a suitable reactor, such as, for example, a paddle drier. However, the mixing can also be carried out in a suitable solvent, such as, for example, water. The mixing step and polymerization step can also be realized in one reactor, however, by simultaneous metering in of the mixture components. The polymerization step can be carried out discontinuously or continuously in suitable reactors. Reactors to be employed discontinuously are, for example, kneading machines or paddle driers.
All high-viscosity reactors which allow removal of the water vapor released, preferably those having large reaction volumes, and preferably also those having kinematic self-cleaning of the surfaces which come into contact with the product and likewise preferably those with shaft heating, can be employed. Machines of corrosion-resistant material, for example stainless steel, are preferably employed for corrosion resistance reasons.
For example, a screw machine of the "Selfcleaner" type from Lurgi can be employed.
The "Selfcleaner" is a continuously self-cleaning screw machine with screws which rotate in the same direction and with hollow shafts through which a heat transfer medium flows for temperature control. The product to be treated is constantly conveyed by the rotating hollow screws through the trough of the apparatus. Above the screw shaft is a gas space for removing the vapors.
The liquid heat transfer medium flows first through the hollow shafts, subsequently enters the last hollow flight close to the product discharge and flows through the hollow flights in countercurrent with the product transported in the trough. The water vapor formed is removed in the product direction.
The hollow screws in the "Selfcleaner" have a self-cleaning circular profile, so that one screw constantly cleans the other. The thread of one hollow screw projects completely, apart from a certain tolerance, into the thread gap up to the hollow shaft of the other screw. The hollow shafts are welded from profiled sheets and as a rule are not finished mechanically.
Products which are difficult to treat and which, under heat treatment, tend to stick and above all to form a crust on the heating surfaces are preferably processed in the "Selfcleaner". The product is transported in the thread gap of the two hollow screws in the form of an open figure-of-eight. The product is mixed in the region of engagement of the hollow screws by the relative speed of the hollow screws.
Other large-volume high-viscosity reactors can also be employed according to the invention, for example of the "AP-Conti" type from List and screw heat exchangers, and polymer screws likewise can also be employed according to the invention. Other types of reactor to be employed according to the invention are the belt reactor (belt dryer) or roller dryer.
The polymerization temperatures are between 100.degree. C. and 300.degree. C., preferably 140.degree. C. to 250.degree. C., particularly preferably 160.degree. C. to 220.degree. C.
The starting material mixtures have a residence time of 0.5 to 300 minutes, preferably 1 to 60 minutes, particularly preferably 2 to 20 minutes, in the reactors described above. The polymerization products are converted into the corresponding polyaspartic acid salt by dissolving in a base at 20.degree. to 95.degree. C., preferably 40.degree. to 70.degree. C., particularly preferably 50.degree. to 70.degree. C. It is also possible for the free polyaspartic acid to be obtained even at this point by hydrolysis in water at 80.degree.-100.degree. C. or by treatment of the salt with acids or acid ion exchangers. The product is obtained as a fine powder by spray drying.
The polymer prepared has different chain lengths and molecular weights, according to analysis by gel permeation chromatography, depending on the reaction conditions, for example the residence time and temperature of the thermal polymerization. (M.sub.w =500 to 10,000, preferably 1,000 to 5,000, particularly preferably 2,000 to 4,000). In general, the content of the .beta.-form in the polyaspartic acid prepared according to the invention is more than 50%, in particular more than 70%.
The compounds according to the invention are used in particular as a dispersing agent, detergent additive, sequestering agent, scale inhibitor, corrosion inhibitor, above all for brass, and as a microbicide.